This invention relates generally to electro-acoustic or audio loudspeaker systems. More specifically, it relates to an apparatus for adjustably filtering an electrical audio signal into a plurality of frequency bands and distributing the audio signal to one or more drivers within a loudspeaker system.
Audio systems present, as an audible signal, a range of audio frequencies. One important characteristic of high-fidelity speaker systems is the relative magnitude response of the speaker system over this audio frequency range. An audio amplifier may provide an electrical input to a speaker system that covers the entire spectrum of audio frequencies that are detectable by the human ear. However, in the present state of the art, a single loudspeaker, or driver, is not capable of accurately reproducing all audio frequencies that are detectable by the human ear. Rather, high fidelity loudspeaker systems have been realized by dividing the audio frequency spectrum into two or more frequency bands and applying each of these bands of the audio spectrum to a separate driver or group of drivers. For example, low frequencies tend to be better replicated by physically larger drivers, commonly known as woofers. Mid-range frequencies, likewise, are more favorably reproduced by a mid-range sized driver. Additionally, higher frequencies are better reproduced by physically smaller drivers, commonly known as tweeters. On the other hand, connecting high-power, low-frequency signals to a tweeter driver, will cause audible distortion and will typically cause fatigue and destruction of the tweeter driver.
To ensure that only the proper frequencies of an electrical audio signal are routed to the appropriate driver, special electrical filters, called crossover networks, have been provided in speaker systems. These networks allow the different drivers or groups of drivers, each adapted for best response to a particular range or band of frequencies, to be combined in a single system capable of wide audio frequency coverage. Thus, the crossover circuit directs the frequency content of the electrical signals over a wide audio range to the appropriate driver or group of drivers in a multi-driver loudspeaker system. Conventional crossover network filter topologies, belong to three classifications according to the frequencies passed and rejected, as follows: (1) low-pass for woofers, (2) band-pass for midranges, and (3) high-pass for tweeters. Where more than one filter is used, the frequency common to adjacent ranges or passbands is called the crossover frequency.
The conventional crossover network design attempts to blend the acoustic output of the multiple drivers to achieve good tonal balance characterized by a smooth transition in acoustic output from one driver to another. One way to accomplish this is a symmetrical crossover network that functions as a filter to assure the response drop-off of one driver as frequency increases through the transition region is a mirror image of the response increase of a companion driver reproducing the adjacent higher frequency band of sound. Proper implementation of this design approach requires that the combination of drivers and crossover networks do not introduce audible artifact (an unnatural sound quality) resulting from frequency response irregularities or phase cancellation effects that potentially result from housing multiple drivers in one speaker enclosure. Thus, conventional wisdom is that a well-designed loudspeaker system exhibits a frequency response in which the output amplitude is relatively flat to desired accuracy (e.g., +/xe2x88x923 dB from 40 Hz to 20 kHz) and phase is linear to desired accuracy. Typically, this frequency response is measured in an anechoic environment, i.e., an environment that is free from echoes and reverberations.
Even if a speaker is designed to have a relatively flat frequency response in an anechoic environment, however, the quality of the sound heard by the listener will vary depending on the acoustical environment in which the speaker system is placed. Thus, the audio characteristics of a given room in which the speaker system is placed can alter the nature and realism of the sound. For example, a room having reflective surfaces will color the sound output of the speaker, resulting in an unnatural sound. Consequently, to enable a speaker system to be tuned to variable acoustical environments in order to accomplish a more natural sound, it is desirable to provide a speaker system having a frequency response that can be adjusted to accommodate the wide diversity of acoustical environments in which the speaker may be placed.
Another important characteristic of a loudspeaker system is its damping factor. Technically, the damping factor of a system refers to the ratio of nominal loudspeaker impedance to the total impedance driving it (amplifier and speaker cable). In practice, damping is the ability of the amplifier to control driver motion once the input signal to the driver has stopped. A high damping factor means that the amplifier""s impedance can absorb the electricity generated by driver coil motion, thereby stopping the driver""s vibration. Damping varies with frequency. The effects of damping are most apparent at low frequencies, in the range of the woofer""s resonance. Speakers that are well damped sound xe2x80x9ctighter,xe2x80x9d while speakers that have a low damping factor result in mushy or indistinct sound. Consequently, it is also desirable to provide a speaker system having a high damping factor.
Another important characteristic of a loudspeaker system is its sensitivity. This characteristic measures the ability of a loudspeaker system to turn electrical energy into acoustical energy. The more sensitive a loudspeaker is, the better it converts electrical energy into acoustical energy. Thus, a more sensitive speaker system is easier to drive. For example, 90 dB sensitivity means that when an amplifier input to the speaker system is set at one watt, the Sound Pressure Level (SPL) output measured at a distance of one meter away from the speaker will be 90 dB, at a given frequency. A speaker that has a 93 dB sensitivity rating, however, will have measured SPL output of 93 dB when the input signal to the speaker is set at one watt.
Still another important characteristic of a loudspeaker system is its dynamic range. The dynamic range is calculated as the difference between the total noise floor (measured in dB(A)) and the equivalent SPL (measured in dB) where a certain amount of total harmonic distortion appears. A speaker system having a greater dynamic range can better handle higher power input, can produce a wider range of sound pressure level output and can reproduce a wider frequency range, all with less distortion.
It is an object of the present invention, therefore, to provide an adjustable loudspeaker frequency distribution apparatus that can be used to easily adjust the frequency output and tonal qualities of a loudspeaker system to accommodate differing acoustical environments in which the loudspeaker system may be placed.
It is another object of the invention is to provide an apparatus that improves the power handling capability and dynamic range of the speaker system.
It is yet another object of the invention is to provide an apparatus that enables the amplifier to better control the speaker voice coil, thereby improving the effective damping factor of the speaker.
Additional objects and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations pointed out in the appended claims.
To achieve the foregoing objects, and in accordance with the purposes of the invention as embodied and broadly described in this document, there is provided a frequency distribution and adjusting circuit that includes an input pair having a positive input and a negative input for receiving an input signal from an amplifier. A low-pass filter has an input electrically coupled to the positive input of the input pair and an output electrically coupled to an input of a low frequency driver, or woofer. A high-pass filter is electrically coupled to at an input of a high frequency driver, or tweeter. A variable resistor network includes a first variable resistance and a second variable resistance coupled in series between a first resistor network terminal and a second resistor network terminal. The variable resistor network also includes a wiper contact with the first variable resistance and a second variable resistance for adjusting the total resistance between the resistive network terminals. The variable resistor network terminals are coupled in parallel with the low-pass filter, and the wiper contact is electrically coupled to the input of the high-pass filter.